During the Continuation War, the Finnish troops in Lapland were astonished how inept the Germans, for all their blitzkriegin' superiority, were in winter warfare. And this is German Gebirgstruppen were talking about.
You'll be hard pressed to find a people more suited to cold weather warfare than the Fins, a people who find Moscow in Winter to be a pleasant place to escape their own meteorological misery.
Anywhoo. The Soviet Offensive idea is certianly not feasible in '39, but by '41 or '42 the Soviet Military should have recovered in part from the purges and have enough material for a medium sized war, especially if they had a smaller war like the Winter War or a Baltic war to reveal some of the cracks in their armor.
If they decide to move into Poland, they'd be in for a considerable fight, but building up for a while should give them the advantage, even if Romania and Czechoslovakia come in on their side. England and France would really like to support the Poles in this situation, but the French people would be very unkind to a war that would sap up manpower and the British would be in an awkward logistical position. This would be exaggerated by the Germans who, while not wanting a boarder with the Soviets, would show distaste for the Allies gaining an ally in Poland. The Germans would remilitarize and even send aid to Poland, but would protest the Allies putting boots on the ground. The remilitarization would also make the French more likely to turtle. (This is assuming a democratic, Capitalistic Germany)
As for Japan, the Russians would certianly win and get Korea and Manchuria. Japan just doesn't have the army to win a war with China and a war with Russia. But the biggest problem with that plan would be China and what would happen after the war. If the Russians enter the war too early the Nationalist Chinese would be strong enough to make a serious protest over the USSR keeping Manchuria, and that could very well spark another (much more costly) war. The Russians would have to wait long enough so the Nationalists are weakened by war and the Communists strengthened by Aid enough so Stalin could either hand Manchuria over to the communists, or keep it while they both brain each other. But again, eventually there would be peace and eventually it would be a very sore issue that would lead to a war the USSR did not want.
There is another option nobody has mentioned yet, and that is Persia. The British would, of course, be outraged because the action would open up India and the Middle East to the USSR. But the problem (from the British perspective) is that there wouldn't be very much they could do about it. If it comes down to a land war between the British and the Russians, the Russians would almost certianly win in a '41, '42 scenario. So in this case, I don't see the British going to war over Persia because if they did, they'd probably lose Iraq and the important Oil Fields there. They would probably pull a Neville, swallow their pride, and try building up as fast as they possibly could before the Russians consolidate their gains and look elsewhere.